The invention relates to a filament wound orthotropic pressure vessel, and is an improvement over that shown in my prior U.S. Pat. No. 3,368,708.
Filament wound vessels have orthotropic load bearing capabilities. This means that the load capacity in one direction is substantially greater than in another direction. For example, a filament wound body can support substantially greater loading in the direction of the fibers than in a direction transverse thereto. In contrast, isotropic material, such as steel, brass and the like, has substantially the same load bearing capability in all three dimensions.
As noted in my prior U.S. Pat. No. 3,368,708, it is desirable to provide a filament wound vessel having a wall stress condition capable of withstanding high internal pressures without weeping. Filament wound vessels are generally fabricated by helically winding a strand of fibrous material impregnated with a thermosetting resin about a cylindrical mandrel in a number of superimposed layers.
When a filament wound vessel is subjected to internal pressure, a tensile stress occurs at the interface between the resin and the fibrous reinforcing material. When the bond between the resin and the fiber fractures, weeping occurs through the vessel wall.
Prior to my said U.S. Pat. No. 3,368,708, one type of cylindrical body vessel wall was typically designed for a 2:1 ratio of hoop stress to axial stress. The winding pattern normally used to meet such stress ratio normally placed two-thirds of the filaments in the hoop direction and one-third of the filaments in the axial direction. In other cases, the fibrous material was wound at a 351/4.degree. helix angle in alternate right hand and left hand helixes with respect to a transverse plane through the vessel. Both of these previous winding patterns were established so that the fibrous reinforcement was loaded in pure tension under the 2:1 stress ratio.
In my prior U.S. Pat. No. 3,368,708, the axial load imposed on the cylinder wall was reduced. The hoop to axial stress condition in the wall was charged from the previous 2:1 ratio to a higher value in the range of 5:1 to 8:1, which in turn withstood a much higher hoop stress before suffering fracture of the resin-reinforcement interfacial bond. The helix angle was in the range of 24.degree. to 191/2.degree. to match the stress ratio condition of 5:1 to 8:1 respectively. The vessel withstood much higher internal pressures without weeping.
The high hoop to axial stress ratio in my prior U.S. Pat. No. 3,368,708 is obtained by one or more longitudinal tie rods taking a portion of the axial load. The rods are secured to one fixed head of the vessel and the opposite ends of the rods are secured to a floating member in the opposite head. The floating member is a central circular section of the opposite head which is connected to the remaining annular portion of the head by a flexible seal which permits relative motion between the circular floating member which is connected to the tie rods and the annular portion of the head which is connected to the cylindrical wall section. When the vessel is subjected to internal pressure, the cylindrical wall expands radially due to the hoop stress, and the radial expansion will tend to shorten the axial length of the vessel. The pressure exerted against the annular section of the head at the end will be imposed as an axial stress in the cylindrical wall which partially offsets the axial shortening, with the result that the opposing head sections move axially toward each other. The pressure acting against the circular floating head member creates a tensile stress in the tie rods such that the fixed head and opposite floating head member move away from each other, and relative movement occurs between the floating member and the annular portion of its head section.
The present invention provides further separation of radial and axial loading and enables even higher hoop to axial stress ratios greater than or equal to about 15:1. The cylindrical body of the vessel is wound substantially only in the hoop direction at a helix angle less than or equal to about 15.degree., without winding around the axial ends of the body. One or more axial continuous filament orthotropic loops carry axial loading and maintain end plates at the open ends of the cylindrical body. The axial length of the body shrinks while the axial loops lengthen. This gives superior performance in high pressure applications.